When misfire is generated while an internal combustion engine is running, an engine rotating speed decreases momentarily. Misfire detection apparatuses detect variation in rotation every explosion stroke of respective cylinders to compare the variation in rotation with a predetermined misfire determination value to determine presence and absence of misfire, as described in JP-7-317586 A.
In some cases, however, a detected value of variation in rotation varies widely even at the time of normal combustion due to dispersion in combustion among cylinders, traveling on rough road, or the like. Alternatively, the detected value of variation in rotation varies widely due to fabrication tolerance in crank angle sensors, which detect an engine rotating speed. Furthermore, there is a situation, in which variation in rotation due to misfire decreases in a region of high rotation, so that these are responsible for degradation in accuracy for detection of misfire.
Also, with the misfire detection apparatus, a target air fuel ratio is corrected toward a rich side for that cylinder, in which misfire is detected, to suppress misfire. And failure in a fuel system or an ignition system is determined in the case where the misfire cylinder is not returned to normal combustion even when correction of a target air fuel ratio toward the rich side is successively repeated predetermined times.
With such construction, however, even when misfire is generated due to failure in an air system, there is caused a problem that failure in the fuel system or the ignition system is erroneously determined. A failure in the fuel system or the ignition system is not determined until correction of the target air fuel ratio toward the rich side is successively repeated predetermined times for the misfire cylinder. Thus, there is a disadvantage that unburned gas subjected to rich correction is discharged from the misfire cylinder until failure is determined and there is caused an increase in HC discharge.
Further, known documents, in which abnormality diagnosis techniques for internal combustion engines are described, include, for example, JP-7-34946 A, in which a technique for diagnosis of abnormality in a fuel system based on a fuel correction quantity by cylinder is described.
Since such abnormality diagnosis technique cannot detect abnormality in an air system, there is involved a problem that abnormality in the air system is erroneously determined to be abnormality in a fuel system. For example, in the case where an intake air quantity increases extraordinarily due to some reason when the fuel system is normal, a fuel increase-correction amount increases extraordinarily so as to maintain a target air fuel ratio, so that there is caused a problem that abnormality in the fuel system is erroneously determined. Besides, it is not possible to detect misfire from a fuel correction quantity by cylinder.
As described above, the prior art involves a problem that accuracy in detection of misfire is low, there is the possibility that the cause for misfire is erroneously determined, and the like.